Page 182 - Primer on Enhanced Oil Recovery

172 Primer on Enhanced Oil Recovery

14.1 Low salinity water injection

Let us say again that the water in the reservoir (connate water) can be divided into: irre-
ducible water (water on the stone surface), free water and clay bonded water. It is possi-
ble to say that almost in every extracted liquid from the well we would have some
amount of water. It makes good economic sense during oil extraction to separate the
hydrocarbons from the extracted water at the extraction site. Separators are used with
great susses for this purpose. After water separation from the oil one needs to deal with a
relatively large amounts of water and somehow dispose it. The separated water, some-
times so named connate water, has very high mineral and chemical content. The
untreated separated from hydrocarbons connate water is very corrosive to the site metal
infrastructures and it is also health and environment hazard if left without the treatment
and/or purification. There are strict regulations, country dependant, on this water disposal.
It was realised more than a century ago that the separated connate water can be
pumped down, re-injected, into the formation. This solves the problem with the
water disposal and at the same time acts as a method for partial reservoir pressure
maintenance. We only use word “partial” as for the full pressure maintenance one
will need more water or to use some gas too. For the last 90 years this re-injection
is widely used in the oil industry.
Generally, water injection is most widely used secondary oil recovery technique.
In many cases this is due to water availability, injection simplicity and low cost and
sufficient efficiency with which water displaces oil. Additionally used, to the con-
nate, water need to be compatible with the connate water and the reservoir chemis-
try. The injected water should be cleaned from bacteria, oxygen and preferably
from many divalent ions. The amount of injected water during waterflooding is
always significantly bigger than the volume of produced oil. This is partially due to
the fact that significant amounts of injected water ends in the production liquids.
The water percentage in the production fluids increases over time as water inevita-
bly finds a way into the production wells. Injected clean, well formulated water
(brine) ends as a complicated cocktail of different dissolved compounds which in
some cases contains even radioactive elements.
Overtime it was realised that the injected water in many cases does much more
than just to mechanically push oil towards production wells. In essence, water by
itself is a chemical agent and alters chemistry of the reservoir. Chemical balance in
the formation shifts and this alters many pre-existing balances. As the result, we
must account for a whole chain of alterations in reservoir equilibria. This re-
balancing proceeds in time and reservoir volume.
Regarding low salinity water flooding, as an EOR technique, requires to view
the injection which reflects positively on oil extraction though shifting chemical
balance in the reservoir. This is instead of use water as just a ramp to move oil in a
secondary oil recovery technique. While positive effects the low salinity water
injection on oil recovery are known from 1940s the mechanism understanding and
appreciation is relatively recent and the process widely recognized and researches
in the last twenty years.